1. Field of Invention
The present invention relates to a method for modifying photoresist images, which are suitable for use in the production of electronic devices such as integrated circuits. More particularly, the invention provides a method for modifying photoresist images by applying a flood electron beam exposure to the resist in combination with optical exposure or patterning.
2. Description of the Related Art
The production of photoresists is well known in the art as exemplified by U.S. Pat. Nos. 3,666,473; 4,115,128 and 4,173,470. These contain aqueous alkali soluble polyvinyl phenol or phenol formaldehyde novolak resins together with light sensitive materials, usually a substituted naphthoquinone diazide compound. The resins and sensitizers are dissolved in an organic solvent and are applied as a thin film coating to a substrate suitable for the particular application desired. The resin component of photoresist formulations is soluble in an aqueous alkaline solution, but the photosensitizer is not. Upon imagewise exposure of the coated substrate to actinic radiation, the exposed areas of the coating are rendered more soluble than the unexposed areas. This difference in solubility rates causes the exposed areas of the photoresist coating to be dissolved when the substrate is immersed in an alkaline developing solution, while the unexposed areas are substantially unaffected, thus producing a positive image on the substrate. The uncovered substrate is thereafter subjected to an etching process. Frequently, this involves a plasma etching against which the resist coating must be sufficiently stable. The photoresist coating protects the covered areas of the substrate from the etchant and thus the etchant is only able to etch the uncovered areas of the substrate. Thus, a pattern can be created on the substrate which corresponds to the pattern of the mask or template that was used to create selective exposure patterns on the coated substrate prior to development.
Photoresists are either positive working or negative working. In a negative working resist composition, the imagewise light struck areas harden and form the image areas of the resist after removal of the unexposed areas with a developer. In a positive working resist the exposed areas are the non-image areas. The light struck parts are rendered soluble in aqueous alkali developers. The ability to reproduce very small dimensions, is extremely important in the production of large scale integrated circuits on silicon chips and similar components. As the integration degree of semiconductor devices becomes higher, finer photoresist film patterns are required. One way to increase circuit density on a semiconductor chip is by increasing the resolution capabilities of the resist. Positive photoresists have been found to be capable of much higher resolution and have almost universally replaced negative resists for this purpose.
The optimally obtainable microlithographic resolution is essentially determined by the radiation wavelengths used for the selective irradiation. However the resolution capacity that can be obtained with conventional deep UV microlithography has its limits. In order to be able to sufficiently resolve optically small structural elements, wavelengths shorter than deep UV radiation must be utilized. The use of UV radiation has been employed for many applications, particularly radiation with a wavelength of 157 nm, 193 nm and 248 nm. In particular, the radiation of lasers is useful for this purpose. The deep UV photoresist materials that are used today, however, are not suitable for 157 nm, 193 nm and 248 nm exposure. Materials based on phenolic resins as a binding agent, particularly novolak resins or polyhydroxystyrene derivatives have too high an absorption at wavelengths and one cannot image through films of the necessary thickness. This high absorption results in side walls of the developed resist structures which do not form the desired vertical profiles. Rather they have an oblique angle with the substrate which causes poor optical resolution characteristics at these short wavelengths. Polyhydroxystyrene based resists can be used in top surface imaging applications in which a very thin (˜500 Å) layer of resist is required to be transparent at ArF laser exposure wavelengths.
Photoresists based on chemical amplification have been developed, which have been found to have superior resolution. Currently available are 157 nm, 193 nm and 248 nm photoresists, which are based on chemically amplified deprotection. With this mechanism, a molecule of photogenerated acid catalyzes the breaking of bonds in a protecting group of a polymer. During the deprotecting process, another molecule of the same acid is created as a byproduct, and continues the acid-catalytic deprotection cycle. The chemistry of a 157 nm, 193 nm and 248 nm photoresist is based on polymers such as, but not limited to, acrylates, cyclic olefins with alicyclic groups, and hybrids of the aforementioned polymers which lack aromatic rings. Chemically amplified resist films have played a significant role in the fine pattern process using deep UV. However, in many cases they lack either sufficient etch resistance, thermal stability, post exposure delay stability, and/or processing latitude. While such photoresists are sufficiently transparent for deep uv radiation, they do not have the etching stability customary for resists based on phenolic resins for plasma etching. A typical chemical amplification photoresist film comprises a polymer, a photoacid generator, and other optional additives. The polymer is required to be soluble in the chosen developer solution, and have high thermal stability and low absorbance to the exposure wavelength in addition to having excellent etch resistance. It would be desirable to overcome the above mentioned problems and to provide a photoresist film superior in etch resistance, as well as transmittance to deep UV.
There have been several attempts to solve this problem. One attempt to improve the etching stability of photoresists based on meth(acrylate) polymers introduced cycloaliphatic groups into the meth(acrylate) polymers. This leads to an improvement in etching stability, but not to the desired extent. Another proposal aims at producing sufficient etching stability only after irradiation in the resist coating. It has also been proposed to treat the substrate with the finished, developed, image-structured photoresist coating with specific alkyl compounds of magnesium or aluminum, in order to introduce the given metals in the resist material as etching barriers (See U.S. Pat. No. 4,690,838). The use of metal-containing reagents, however, is generally not desired in microlithography process, due to the danger associated with contamination of the substrate with metal ions.
U.S. Pat. No. 6,319,655, which is incorporated herein by reference, describes a process for improving the etch resistance of chemically amplified resists, in particular 193 nm sensitive photoresists, using a large area electron beam exposure. Electron beam exposure of chemically amplified photoresists, in particular 193 nm sensitive photoresists has been shown to improve the etch resistance and thermal stability of these photoresists. Many different formulations of chemically amplified photoresist utilized for 193 nm exposure have been developed. Some examples of materials used for 193 nm lithography are given in patent No. U.S. Pat. No. 6,319,655. For the next generation of lithography, new resist materials sensitive to 157 nm irradiation will be utilized for this application. Some of these materials, incorporated herein by reference, are listed in “Organic Imaging Materials, A View of the Future” by Michael Stewart et al., Journal of Physical Organic Chemistry, J. Phys. Org. Chem. 2000; 13: 767-774, “157 nm Resist Materials: Progress Report” by Colin Brodsky et al., J. Vac. Sci. Technol. B 18(6), Nov/Dec 2000, 3396-3401, and in “Synthesis of Siloxanes and Silsesquioxanes for 157 m Microlithography” by Hoang V. Tran, et al, Polymeric Materials: Science & Engineering 2001, 84. Due to the volatility of the additives in these resist materials, electron beam exposure causes the expulsion of these additives, which causes shrinking of the resist. For 248 nm, 193 nm, and 157 nm resist technologies, chemical amplification is used to achieve high photo speed at the low exposure energies of the selected wavelength sources. For future lithography generations using extreme ultraviolet and x-ray wavelengths of 1-100 nanometers, similar chemically amplified resists are anticipated. The basic resist design concept is to start with a resin that has good transparency at the selected wavelength. The resin must be highly soluble in aqueous base developer chemistry. To make the resin insoluble in the developer, dissolution inhibitors, sometimes called blocking or protecting groups, are attached to the resin. These are usually very large, or bulky, molecules that are attached to the resin via bonds that can be easily cleaved. In most advanced resist systems there are usually several types of molecules attached to the resin in addition to the dissolution inhibitor. These include molecules that enhance the etch resistance of the material as well as molecules that help with lithographic performance. All of these molecules are attached to the resin via a link that is easily cleaved. The chemical amplification is achieved by adding a small amount of a photo-acid generator (PAG). This is a compound that generates a proton (H+) when exposed at the appropriate wavelength. These are usually onium salts, such as sulfonium salts, but it can be any of a number of suitable compounds. When the PAG is exposed and the proton is generated, the proton cleaves the nearest available bond between the resin and dissolution inhibitor. This cleaving reaction generates another proton, which cleaves the next nearest bond, and so on. This reaction can occur during the optical exposure, for low activation energy resists, or during the subsequent post-exposure bake (PEB), for high activation energy resists. The result of the de-protection reaction is the formation of an acid, which is then soluble in an aqueous base developer. As a result of the cleaving of the link between the resin and the blocking group, the blocking molecule usually leaves the resist as a volatile. In this way the resist can be fully exposed even though the incident optical exposure dose is very low, on the order of 10 to 20 mJ/cm2.
After the optical exposure and completion of the de-protection reactions, the resist in the exposed areas can shrink from ten to twenty percent. This is due to the loss of the bulky protecting groups as volatiles. This reaction does not happen in the unexposed areas, which provides the contrast to form the images. Since the unexposed resist still contains the resin with the attached blocking groups, it is susceptible to shrinkage if these molecules are removed.
Due to the constraints of the resist design, and since the blocking groups are easily cleavable, the blocking groups can be removed by other means. One reaction path is thermal activation, where the resist is heated to a temperature that thermally breaks the bonds. This happens at different temperatures for the different protection groups but can be a low as 40° C. to as high as 200° C. Thermal activation results in the loss of the blocking groups as volatiles and a shrinkage of the resist. The blocking groups can also be removed by other radiation sources including plasma discharges, or accelerated particles.
During electron beam exposure, a reaction that is similar to the optical exposure can occur which cleaves the link between the protecting groups and the resin resulting in shrinkage of the resist. This reaction, and the associated resist shrinkage, is accelerated as the resist is heated by the energy of the incident electron beam. Since the full thickness of the resist is targeted for stabilization, substantial mass loss, and shrinkage, can result from the electron beam exposure. Because the interface between the resist and substrate is constrained, the remainder of the resist shrinks in three dimensions. This leads to a phenomenon known as “pullback” where the top of the resist shrinks relative to the bottom. This effect is most pronounced on lithographic features such as contacts, line ends, and feature corners. The pullback phenomenon has undesired effects on the features, which make them unacceptable for device fabrication. This shrinkage occurs throughout the exposed regions of the photoresist and can cause deformation in the form of pullback on the upper portions of lithography features.
Many attempts have been made to correct or eliminate this resist deformation or pullback by using different process steps with the electron beam exposure. Lower current density exposures have been attempted to minimize the shrinkage as well as surface curing of the photoresist, that is, lowering the energy of electrons such that only the upper portion of the resist receives the electron beam exposure. Higher doses of electrons have been utilized at the lower portion, relatively to the upper portion, of the resist in an effort to minimize this pullback. In addition, different formulations of photoresist have been attempted to minimize the shrinkage and expulsion of resist components to minimize the pullback. Lower flux electrons with longer exposure times have also been utilized to minimize resist heating effects thereby reducing the temperature of the resist during electron beam exposure to no more than 50° C. All of these attempts have failed at reducing the pullback effect caused by the electron beam hardening process.
Furthermore, additional issues such as proximity effects, acid diffusion, and aerial image quality, effect the resulting features printed in resist films. These issues limit the ability to print the desired patterns for manufacturing. Issues such as line slimming and etch resistance effect the subsequent processing of the patterned resist. Line slimming, which is more pronounced with chemically amplified resists, and especially with ArF or 193 nm resists, is a phenomenon where the resist shrinks when it is being measured by a CD-SEM. Thus the features change in shape while they are being measured. This impacts the ability to determine the exact result of the lithography process as well as the ability to monitor the process for variations. The lack of etch resistance of the materials limits the ability of the resist to provide an adequate mask for pattern transfer. This effects the resulting etched pattern, and ultimately the performance characteristics of the devices being manufactured. In co-pending U.S. patent application Ser. No. 10/090,465, filed on Mar. 4, 2002, which is incorporated herein by reference, a technique is described for reducing the pullback caused by lateral resist shrinkage during e-beam stabilization by cooling the wafer during e-beam irradiation.
It has now been surprisingly found that by exposing the photoresist to flood electron beam exposure after optical or other patterned exposure but prior to the develop step, that the pullback or lateral resist shrinkage on the upper region of lithographic images in resist can be significantly reduced during electron beam processing. This unexpected result is due to the fact that the flood electron beam exposure and optical or other patterned exposure are applied in combination, and are carried out prior to development of the resist. This means that the resist shrinkage that is seen as a result of these steps is constrained laterally by the resist film itself. Thus, the resist is free to shrink vertically, and the resulting shrinkage provides a reduction in the line slimming and an improvement in the etch rate of the resist without significant pullback or rounding of the resist images This leads to the formation of a better resist image.